Spirometry

• Tidal volume is the volume of air inspired or expired per normal breath (500mL)
• Inspiratory reserve volume is the volume of air inhaled by forced maximal inspiration at the end of normal inspiration (2000-3200mL)
• Expiratory reserve volume is the maximum volume of air that can be expired from lungs at the end of normal expiration (750-1000mL)
• Residual volume is the volume of air remaining in lung after forced expiration (1200mL)
• All the lung volumes combined (TV, IRV, ERV and RV) = total lung capacity

  Total lung capacity normal or increased in obstructive disease

  Total lung capacity reduced in restrictive disease
• Functional residual capacity = expiratory reserve volume + reserve volume

  Everything left in the lungs after normal expiration
• Inspiratory reserve capacity = tidal volume + inspiratory reserve volume
• Vital capacity = inspiratory reserve volume + tidal volume + expiratory reserve volume

  Patient takes a deep breath in a breathes out everything they can

  Reduced in restrictive disease
• Patients with obstructive disease will have an increased residual volume = air trapping
• On a chest x-ray you can normally only see the 7th-9th anterior ribs. Visibility of lower ribs suggests hyperinflation - seen in COPD
• Examples of obstructive lung disease:

  • COPD
  • Asthma
  • Bronchiectasis
  • Cystic fibrosis (mucus)
  • Post TB
  • Lung cancer
• Examples of restrictive lung disease:

  • Fibrosis
  • asbestosis
  • Sarcoidosis
  • Extrapulmonary: rib deformity, obesity and pregnancy
• Restrictive lung diseases cause stiffness which resists inspiration therefore all standard lung volumes and capacities therefore reduced
• Key spirometry measurements:

  • FEV1 - forced expiratory volume in one second
  • FVC - total volume of air that can be forcibly exhaled in one breath
  • FEV1/FVC ratio - the fraction of air exhaled in the first second relative to the total volume exhaled
• To assess reversibility, administer 400 micrograms of salbutamol and repeat spirometry after 15 minutes:

  • The presence of reversibility is suggestive of a diagnosis of asthma.
  • The absence of reversibility suggests fixed obstructive respiratory pathology such as chronic obstructive pulmonary disease (COPD).
  • Partial reversibility may suggest a coexisting diagnosis of asthma and another obstructive airway disease (e.g. COPD).
• Obstructive lung diseases will have a FEV1/FVC ratio of <0.7

  The FVC is reduced but not as much as it is in restrictive diseases

  There is a prolonged time to full expansion and reduced volume expired
• Restrictive lung diseases will have a reduction in all parameters and therefore will have a normal FEV1/FVC ratio of >0.7

  • Reduced FEV1 (<80% of the predicted normal)
  • Reduced FVC (<80% of the predicted normal)
• In obstructive lung disease with features of air trapping, the residual volume may be significantly increases. Values in excess of 140% of the total lung capacity significantly increase the risk of pulmonary hypertension, pneumothorax and infection